Python Strings and Data Structures Learning Objectives Strings - - PowerPoint PPT Presentation

Python Strings and 
 Data Structures

CS 6452: Prototyping Interactive Systems

Learning Objectives

• Strings (more)
• Python data structures

− Lists − Tuples − Dictionaries

• Get comfortable writing more code

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CS 6452: Prototyping Interactive Systems

Questions?

• Basic Python OK?
• How was the HW?

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CS 6452: Prototyping Interactive Systems

Strategies

• Don’t write up your entire program all at
• nce
• Decompose it into pieces & get each piece

working independently

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CS 6452: Prototyping Interactive Systems

Multiple values

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def mult3(a, b, c):
 return a+1, b+2, c+3
 
 a, b, c = mult3(1, 1, 1)

CS 6452: Prototyping Interactive Systems

Strings

• Used everywhere


(Take out your laptops)

• >>> s = “Hey!”


>>> print(s + “ You”)
 >>> print(len(s))
 >>> print(s * 3)

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CS 6452: Prototyping Interactive Systems

Printing Elements

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for letter in “Hello”:
 print(letter)
 
 str = “run”
 for ch in str:
 print(ch, end=‘ ‘)

Print only vowels?

str = “dictionary”
 for letter in str:
 if letter in “aeiouAEIOU”:
 print(letter)

Print all the letters in a string

CS 6452: Prototyping Interactive Systems

Reverse

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How to reverse a string? def reverse(str):
 result = “”
 for letter in str:
 result = letter + result
 print result

CS 6452: Prototyping Interactive Systems

Indices

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Strings have indices W i n e n r

[0] [1] [2] [3] [4] [5]

str = “Winner”
 print(str[4])
 
 print(str[-1])
 print(str[-2]) print(str[6])
 
 print(str[1:3])

CS 6452: Prototyping Interactive Systems

Alt Traversal

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Traverse, print, and reverse characters with while, not for def reverse2(str):
 index = 0
 rev = “”
 while index < len(str)
 print(str[index]) rev = rev + str[index]
 print(rev)

Wrong

CS 6452: Prototyping Interactive Systems

Alt Traversal

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Traverse, print, and reverse characters with while, not for def reverse2(str):
 index = 0
 rev = “”
 while index < len(str)
 print(str[index]) rev = str[index] + rev
 index = index + 1
 print(rev)

CS 6452: Prototyping Interactive Systems

Modify a String?

• Strings are immutable

− Once created, cannot be changed

• So how do you “modify” one?
• Always create a new one

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CS 6452: Prototyping Interactive Systems

String Operations

• Many functions on strings

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s.count(s1) – count of how often s1 occurs in s
 s.find(s1) – Returns first index of s1 in s (-1 if not there) s.lower() – convert to lowercase s.upper() – convert to uppercase s.replace(old, new) – replaces all occurrences of old with new s.isalpha() – true if only contains alphabetic characters s.isdigit() – true if only numbers s.lstrip() – removes leading whitespace from s s.rstrip() – removes trailing whitespace from s s.strip() – removes leading & trailing whitespace from s s.isupper() – true if all uppercase


… Remember: Some return a new string, don’t modify existing one

CS 6452: Prototyping Interactive Systems

Useful function

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>>>str = “ John plays golf”
 >>>l = str.split()
 >>>print(l) [‘John’, ‘plays’, ‘golf’]
 
 
 
 >>> str.strip().lower().split() ???


A list (more to come soon)

CS 6452: Prototyping Interactive Systems

Parsing a String

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str = “From: Bruckman, Amy S asb@cc.gatech.edu Date: Fri, 26 Aug 2016 20:32:17 +0000” 
 pos = str.fund(‘@’)
 space = str.find(‘ ‘,pos)
 host = str[pos+1,space]

From: Bruckman, Amy S asb@cc.gatech.edu Date: Fri, 26 Aug 2016 20:32:17 +0000

Want second half of email (after @ sign) in this

CS 6452: Prototyping Interactive Systems

Exercise

• Create a palindrome tester

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def palindrome(str):
 start = 0
 end = len(str) – 1
 while start < end: if str[start] != str[end]
 return False
 start = start + 1
 end = end - 1
 return True

CS 6452: Prototyping Interactive Systems

Helpful Stuff 1

• dir function – lists all methods on a type
• f object

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>>> stuff = 'Hello world' >>> type(stuff) <type 'str'> >>> dir(stuff) ['capitalize', 'center', 'count', 'decode', 'encode', 'endswith',
 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower',
 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'partition',
 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split',
 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

CS 6452: Prototyping Interactive Systems

Helpful Stuff 2

• help function tells what a method does

18 >>> help(str.capitalize) 
 Help on method_descriptor: 
 capitalize(...) 
 S.capitalize() -> string 
 Return a copy of the string S with only its first character capitalized.

CS 6452: Prototyping Interactive Systems

Admin Intermission

• Survey
• Piazza
• Office hours
• Slides
• Code in t-square

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CS 6452: Prototyping Interactive Systems

Data Structures

• Sometimes, you need more than a variable

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CS 6452: Prototyping Interactive Systems

Variables

• A variable is simply a name that contains a reference to

some information

• foo = “Jim”
• Variables can be reassigned, and multiple variables can

refer to the same thing

• Stashing a reference in a variable gives you a way to

name it, and get at it later

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foo “ “Jim”

CS 6452: Prototyping Interactive Systems

Problem

• Some more complex structures are hard to represent by

just a named variable though

• Example: you want to keep track of all of the users in a

chat

− user1 = “Steven” − user2 = “Amy” − ...

• This is too static. Would you just create 1000 variables in

case you ever had that many users? How would you do something to each one (can’t easily iterate)

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CS 6452: Prototyping Interactive Systems

Lists to the Rescue

• Fortunately, Jython has a built in way to do this: lists
• foo = [ “one”, “two”, “three” ]
• Lists collect multiple references to data items into a single data

structure

• These references are ordered
• The contents of the list can be altered (it is mutable)
• currentChatUsers = [ “Amy”, “Steven”, ... ]

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foo List

CS 6452: Prototyping Interactive Systems

List

• Sequence of values
• Heterogeneous (not all same type of

value)

• Mutable!
• Denoted with [ ]


 [50, 40, 30, ‘Mary’, ‘Fred’]

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CS 6452: Prototyping Interactive Systems 25

evens = [2, 4, 6, 8]
 names = [“Jim”, “Jane”, “Mike”, “Mary”]
 vals = range(5) # vals is [0, 1, 2, 3, 4]
 nums = range(1,10,3) # ???
 for i in nums:
 print(i)

CS 6452: Prototyping Interactive Systems

Accessing Elements

• [ ] used to get an index

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days = [‘sun’, ‘mon’, ‘tue’, ‘wed’, ‘thu’, ‘fri’, ‘sat’]
 c = days[3] print(c) print( days[-1] ) week = days[1:6]
 print(week) days[2] = ‘sleep’
 # What happens?

Mutable

CS 6452: Prototyping Interactive Systems

List Methods

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append(item) – Adds item to end of list
 count(item) – Returns count of how often item appears
 index(item) – Returns index of first element with value item
 insert(index, item) – Put item into list at position index and


slide all others over one to the right


sort() – Sort items so they appear in ascending order
 remove(item) – Remove first occurrence of item
 reverse() – Reverses order of list >>>l = [‘a’, ‘b’, ‘c’]
 >>> del l[1]
 >>> print(l)

CS 6452: Prototyping Interactive Systems

Aliases

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list1 = [1, 2, 3, 4]
 list2 = list1
 list1[2] = 12
 
 print(list1)
 print(list2) list3 = [] + list2
 list3.append(10)

CS 6452: Prototyping Interactive Systems

Tuple

• Like lists, only immutable

− The set of references in a tuple is fixed

• Generally used either when:

− You need a constant list

daysOfWeek = ( “Monday,” “Tuesday”, “Wednesday”, “Thursday”, “Friday”, “Saturday”, “Sunday” )

− You need to group together a set of data whose structure is fixed:

E.g., using tuples as quick-and-dirty records, such as address book entries:

myContactInfo = ( “John Stasko”, “TSRB355”, “stasko@cc.gatech.edu” )

• All list operations work on tuples, except ones that modify

the set of references within the tuple

− So, no append(), remove(), etc.

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CS 6452: Prototyping Interactive Systems

Tuple

• Immutable!
• Lists of comma separated values

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t1 = ‘a’, ‘b’, ‘c’
 t2 = (‘a’, ‘b’, ‘c’) # equivalent
 
 t3 = tuple(‘bobcat’)
 print(t3)
 t4 = (10, 20, 30, 40)
 print(t4[2])
 print( t4[0:2])

CS 6452: Prototyping Interactive Systems

Access

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>>> m = [ ‘go', 'fish' ] >>> (x, y) = m >>> x ‘go' >>> y 'fish' >>>
 
 
 >>> b,a = a,b What does that do?

CS 6452: Prototyping Interactive Systems

Multiple values

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def mult3(a, b, c):
 return a+1, b+2, c+3
 
 a, b, c = mult3(1, 1, 1)

Recall

CS 6452: Prototyping Interactive Systems

Associating Data Items

• Sometimes, you need to associate one item with

another one

− Example: hours worked on each day of the week:

• You could do this with variables, as long as

there’s a fixed set of them:

− sunday=4.5 − monday=8

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“Sunday” 4.5 “Monday” 8 ... ...

CS 6452: Prototyping Interactive Systems

Associating Data Items

• If you don’t know the associations you might have

up front, you could use parallel lists:

− workDates = [ “1/29/05”, “1/30/05”, “2/1/05”, ... ] − workHours = [ 4.5, 8, 5.5, ... ]

• Then, iterate through the first list to find the date

you’re looking for, then look for the item with the corresponding index in the second list

• Too much work! Too error prone!
• Fortunately, Python has a built-in data structure

for creating associations: the dictionary

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CS 6452: Prototyping Interactive Systems

Dictionary

• Like a list, but the index can be anything

− You state what it is − Called a key

• Made up of key,value pairs
• Used to store and subsequently access

data

• Similar to a hash table

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CS 6452: Prototyping Interactive Systems

Example

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902634854, “Sally Wilson” 917365643, “Lars Jonsson” 931967385, “Sakshi Gupta” 923438961, “Jiang Xiao” … Syntax: { key1:val1, key2:val2, … }

CS 6452: Prototyping Interactive Systems

Dictionary Data Structure

• Dictionaries associate values with keys (you lookup a value given its key)
• Both are references to data items
• workRecord = {“1/29/05”:4.5, “1/30/05”:8, “2/2/05”:5.5 }
• Dictionaries are the most commonly used Python data structure
• Virtually any Python data types can be used as a key or value

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workRecord

Dictionary

“1/29/05” 4.5 “1/30/05” 8 “2/2/05” 5.5

CS 6452: Prototyping Interactive Systems

Code Example

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months = {‘Jan’:1, ‘Feb’:2, ‘Mar’:3, 1:’Jan’, 2:’Feb’, 3:’Mar’}
 
 print(months[2])
 print(months[‘Jan’]) print(months.keys())
 print(months.values())

CS 6452: Prototyping Interactive Systems

Important Note

• It is not ordered, ie, order is unpredictable



 print(months)

• What happens?

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CS 6452: Prototyping Interactive Systems

Walking through

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total = {‘dave’:83, ‘sue’:91, ‘audrey’:77}
 for key in total: print(key, total[key])

How might you print them in sorted (alpha) order?

total = {‘dave’:83, ‘sue’:91, ‘audrey’:77}
 print(total)
 lst = list(total.keys())
 lst.sort()
 for key in lst: print(key, total[key])

CS 6452: Prototyping Interactive Systems

Exercise

• Want to write a program that, given a big

string, counts how often each letter appears

• How do it?

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CS 6452: Prototyping Interactive Systems

Solutions

• 1. Make 26 variables

− Yuk

• 2. Make a list

− Need numeric index

• Take ordinal value of character as index

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CS 6452: Prototyping Interactive Systems

Solutions

• 3. Use dictionary

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word = ‘areallongword' d = dict() for c in word: if c not in d: d[c] = 1 else: d[c] = d[c] + 1 print(d)

CS 6452: Prototyping Interactive Systems

Operations Summary

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d[k] – returns item in d with key k
 len(d) – returns number of items in d list(d.keys()) – returns a list containing the keys in d list(d.values()) – returns a list containing the values in d k in d – returns true if key k is in d del d[k] – removes the key k from d d.get(k,v) – returns d[k] if k is in d, and v otherwise d[k] = v – associates value v with key k in d


(replaces an existing value, if present)

for k in d – iterates over keys in d
 d.items() – returns a list of (key,value) tuples

…

CS 6452: Prototyping Interactive Systems

Learning Objectives

• Strings (more)
• Python data structures

− Lists − Tuples − Dictionaries

• Get comfortable writing more code

45

CS 6452: Prototyping Interactive Systems

Next Time

• Manipulating files

− Reading and writing

• Starting to work with data

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